1. Field
This invention relates to light limiters which protect optical detectors or sensors (including eyes). It is well known that unprotected optical sensors can be damaged by exposure to intense laser radiation. Protectors or optical limiters may be of two general types, active or passive. Active optical limiters require a predetermination of the presence of threat radiation and then must provide an external stimulus (such as an applied electric field) to operate the limiter. Active devices are usually complex (tunable filters for example) and are unable to respond to short pulses or bursts of harmful radiation. Passive protectors are preferred since the threat radiation itself triggers the desired protective response.
An ideal protector must fulfill various requirements: it must not degrade or attenuate desired radiation (low insertion loss); it must provide complete blocking of radiation above some predetermined detector damage threshold (harmful to the sensor or eyes), it must be sensitive over a sufficiently wide wavelength range to block all undesired radiation, it must have a wide field-of-view, it must (in many cases) be fast acting, and it must be capable of simultaneously blocking a multiplicity of intense laser wavelengths emanating from a single source.
Of late, there has been considerable interest in protecting sensors and eyes from high-power multi-line laser sources (directed energy) which are currently under development. The basic design of such lasers include a neodyminum-doped laser host material such as glass or yttrium aluminum garnet (YAG). The fundamental output wavelength of such lasers is at 1.05 to 1.06 micrometers depending on the particular laser host material. This fundamental wavelength is frequency doubled (wavelength is halved) to approximately 0.53 micrometers and the beam at this wavelength is used to pump a Raman cell which, in turn, emits several additional laser frequencies which are typically at several hundreds of Angstroms from the pump wavelength. These laser systems (known as "rainbow" or "white light" lasers) are designed to deliver all of these laser lines simultaneously. That is, four or five laser lines at different wavelengths (each are separated by several hundered angstroms in the visible spectrum) will arrive at a targeted sensor simultaneously. In addition, the multiplicity of laser wavelengths (each at substantial intensity) are capable of being delivered to the system in short pulses (10 nanoseconds) at a repetition rate of 10 Hz or in a number of random delivery rates.
2. Prior Art
This threat poses a serious problem for existing sensor-protection technology. Fixed-line filters are not viable solutions since the exact wavelengths of the Raman shifted 0.53 micrometer pump laser are uncertain and could even be changed during operation of the laser. Another drawback of fixed-line filters for so many laser lines is the large insertion loss. This particular problem applies to state-of-art "rugate filters" as well, that is, if more than three rejection wavelengths are designed into the filter, the overall transmission of the filter is seriously degraded which in turn impaires the sensor mission. Tunable filters are not capable of countering such a threat for two reasons: they cannot respond to nanosecond pulses and they cannot filter a multiplicity of laser lines simultaneously.
An object of the present invention is to overcome these problems and provide passive broadband sensor protection against high-intensity, short-pulse, high repetition rate multi-line lasers. The teachings of the invention can be used to passively protect night-vision devices, sensitive photodetectors and eyes.